Progressive multifocal contact lens suitable for compensating presbyopia

ABSTRACT

The progression of the optical power of the lens comprises a first tract (a) that descends rapidly to the base dioptric power and in which the lens functions for near vision; a short second tract (b) that remains substantially at the level of the base dioptric power and in which the lens functions for far vision; and a third tract (c) consisting of a first progressively rising part (c′) and a constant second part (c″) and in which the lens functions progressively for intermediate distance in the first part and in a constant manner for near vision in the second part.

BACKGROUND OF THE INVENTION

The present invention relates to a progressive multifocal contact lenssuitable for compensating presbyopia or, more generally, an ametropiaassociated with presbyopia.

The state-of-the-art multifocal contact lenses for the purposes set outabove comprise physically defined optical zones for generating a doublefocalization of a single object.

In particular, GB 2288033 A reveals a contact lens for correctingpresbyopia in which the central part is aspherical and corrects the nearvision, while the peripheral annular part is spherical and corrects thefar vision, where the aspherical part is multifocal, while the sphericalpart has a single focus.

The principal drawback of the known lenses is that a sufficiently validvision is obtained only if and when the lens achieves and maintainsadequate dynamics, that is to say, an adequate movement on the surfaceof the cornea. But even when this result is obtained, it will beaccompanied by a fractionation of the radiant energy flux, which iseffectively divided by the two areas of different dioptric power with aconsequent loss of visual capacity of the wearer of the lens as regardsmesoptic and nocturnal vision.

SUMMARY OF THE INVENTION

A long series of experiments has led to the present invention, which iscapable of obviating these drawbacks and providing advantages that willbe explained in due course. The invention is a progressive multifocalcontact lens that makes the optical power vary with respect to the basedioptric power from the centre to the periphery of the entire opticalzone, the power variation being such that it first diminishes, thenremains constant and eventually rises again; this lens is capable ofcompensating the optical effect of presbyopia or other anomalies of theaccommodation system.

The inventors deem that the new and unforseeable results obtained bytheir experiments could find explanation in the capacity of the combinedsystem constituted by the eye and the brain of selecting the mostsuitable focal point for furnishing a clear perception of what theviewer wants to see within the scenario that engages his attention.Starting from what is currently known about the image perceptionmechanism, the inventors have- as it were- followed the path of thelight rays backwards from the retina to the external surface of thecontact lenses. It was simulated with the help of a computer and, usingsoftware specially created for this purpose, the optical zone of thecontact lens was then processed and re-elaborated to define anuninterrupted bundle of clearly perceptible focal zones; this led to theconstruction of a rather complex surface, but which furnishes a farvision exactly equal to that of a normal monofocal contact lens and anexcellent vision at both intermediate distances (desk, computer)and neardistances (books, wristwatch, etc.)

The present description employs a number of terms that are to beconventionally understood as having the following meanings:

(i)—lens—stands for contact lens;

(ii)—internal—and—external—, when referred to the surfaces or theprofiles of a lens, stand respectively for the side in contact with theeye and the side facing away from it;

(iii)—progression—indicates the fundamental characteristic of theinvention: it is the function according to which the optical power ofthe lens varies with the distance from the centre and right through tothe periphery of the lens;

(iv)—profile—of the lens is used to indicate the profile, generally theexternal one, that is presented by a radial section of a lens (theterms—profile—and—external surface, or internal surface—may at times beused indifferently to express one and the same concept;

(v) the—base dioptric power—is used herein for what is also known as theneutral power or the value of the ametropia or the far vision power ofthe lens;

(vi) the diagramme in which the progression of the lens is to berepresented has the distance from the centre to the periphery of theoptical zone of the lens as its abscissa and the optical power of thelens as ordinate (it will be the diagram of a rotation figure around thecentre of the lens);

(vii) the expressions—to see from close by—and—to see from far away—aresimplified into respectively—for near vision—and—for far vision—.

The progression has the characteristic of presenting three tracts, eachjoined to its immediate predecessor: a) a first tract in which the lensfunctions for near vision, which commences from a peak at the centre todecrease very rapidly to the base dioptric power; b) a second tract inwhich the lens functions for far vision and remains substantiallyparallel to the abscissa at the level of the base dioptric power; c) athird tract comprising a first progressively increasing part and asecond part parallel to the abscissa, corresponding to the powers atwhich the lens functions progressively in the first part forintermediate distances and in the second part in a constant manner fornear vision.

It has been found that the first and the third tract, a) and c),separated by the second tract, b), influence each other and are bothindispensable for furnishing an adequate vision for intermediatedistances and for nearby.

It can be readily understood that the progression creates a series ofconcentric optical zones on the surface of the lens: a first central andcircular zone and others in annular form of greater or lesser width inthe radial direction, each of the latter obviously having equal opticalcharacteristics throughout its radial and circular extension, and it canalso be seen that, for each person suffering from a particularrefraction status, the progression, though always characterized by theaforesaid three tracts, will have a peculiar pattern of its own,generally different from all others.

The lens of the invention can be either of the soft, hydrogel or rigidgas-permeable type.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in greater detail and makingreference to the attached drawings, wherein:

FIG. 1 shows a logic scheme of the construction of the external surfaceof a lens;

FIG. 2 shows a diametral lens section associated with a schematicdiagram;

FIG. 3 shows a first detailed diagram;

FIG. 4 shows a second detailed diagram; and

FIG. 5 shows a group of six diagrams.

DESCRIPTION OF PREFERRED EMBODIMENT(S)

FIG. 1 shows the succession of phases for arriving at the definition ofa lens in accordance with the invention.

i) the choice of the base curve, or internal curve, that has to beadapted to each specific eye is determined by the application choices ofthe optometrist; since it constitutes a variable, as many radii ofcurvature are found as there are mathematically possible curves and itdoes not therefore constitute a characteristic property of the lens inquestion;

(ii) the base dioptric power of the lens is a function of the refractionstatus for far vision of the subject under consideration and,consequently, is once again not necessary for compensating thepresbyopia;

(iii) having defined the base curve and, if necessary, also the basedioptric power, a particular power variation is imposed on the externalsurface of the lens and extending over the entire optical zone for thepurpose of compensating the presbyopia. This power variation becomestranslated into a continuous variation of the profile or externalsurface. Though using only one of the possible specific progressions(this condition will be more readily understood later on), as manyprofiles will be obtained as there are combinations deriving from theassociation of base curves and dioptric power for far vision. Theprogression determines the capacity of this lens of correctingpresbyopia. Thus, the progression in the lens will be determined.

(iv) the progression is thus a function of the previously describedelements; the possible combinations generate millions of profiles, alldifferent from each other, and a given lens will be equal to anotheronly if the constituent elements of both lenses (i.e. base curve, powerfor far vision, and power variation from which the external curve isobtained) are all identical. Realization of the external surface isobtained with a conventional machine suitable for processing contactlenses and controlled by an express computer program.

FIG. 2 shows the diametral section of a lens 1; the progressivevariations of the profile of the lens are of the order of hundredths ofa millimetre and take place every ten microns from the lens centretowards the periphery of the optical zone. These variations areimpressed upon the lens in accordance with the following rule: for everysmall area from the centre of the lens, for every basic ray needed forthe application of the lens to the eye and for every fraction of adioptre for far vision starting from zero there first exists an aureoleof circular form and then other aureoles of annular form, and theprofile of each of these has a radius of curvature of its own; it istherefore readily apparent that the possible profiles of the lensaccording to the invention are unlimited; for this reason it is notrealistic to try to represent any one of the profiles of the lens in afigure; what has rather been done here is to superpose onto the profileof the lens the schematized diagrame of the progression in the formof—optical power/distance from centre—diagrame that guided theconstruction of the lens. The diagrame applied to the lens profile showsin schematic form the three tracts a), b) and c) that characterize theprogression. To the right of the lens 1 there is shown a series ofpoints A on the optical axis I—I representative of the very large numberof focalization points produced by the variable optical power of thelens. The fact that the progressive variations of the lens profile aremicroscopic ensures that the progression may be imparted onto theinternal surface of the lens rather than its external surface; indeed,such microscopic variations do not disturb the surface of the cornea.

FIG. 3 shows a progression example in some detail, that is to say, oneparticular manner in which the optical power of the lens may varybetween the centre of the lens and the perimeter of the optical zone;the tract a) has a peak about 5 dioptres at the centre of the lens andthen descends rapidly to have its foot at the value of 0 dioptres,equivalent to the optical power for far vision, at a distance of 0.7 mmfrom the centre of tile lens; the tract b) is parallel to the axis oftile abscissa at the level of 0 dioptres and remains substantiallyconstant from the foot of the tract a), i.e. from 0.7 mm to 1.8 mm fromthe centre of the lens; the tract c) consists of a first part c′),adjacent to tract b), which progressively rises from the aforesaid valuepair 1.8 mm-0 dioptres to the value pair 3.0 mm-4 dioptres, and a secondpart c″), which remains substantially constant at the value of 4dioptres for the whole of the remaining extension of the optical zone ofthe lens, i.e. from 3.0 to 9.0 mm.

FIGS. 4 and 5 show that the progression illustrated by FIG. 3 is nothingbut a particular case of one of the families of progressions peculiar tothe lens in accordance with the invention; FIG. 4 shows three parts a,b, c that indicate, respectively, the region of the diagram in whicheach of the tracts a), b), c) of the infinity of progressions may cometo be situated. FIG. 5, on the other hand, shows six possibleconfigurations (5/1-5/6) that may be assumed by the progression. Thisfigure shows only the reference numbers on the axes of the abscissas andthe ordinates, but does not identify the tracts a), b) and c), which arethe same as those indicated in FIG. 3. The configurations reproduced inFIG. 5 show as many possible progressions as are comprised in thediagrame of FIG. 4, which is not to be understood as limitative.

What is claimed is:
 1. A progressive multifocal contact lens (1)suitable for compensating presbyopia, characterized in that the power ofthe optical zone varies continuously with respect to the base dioptricpower between the centre of the lens and the periphery of the opticalzone according to a progression presenting:—a peak at the centre, fromwhich there departs a first tract (a) that rapidly descends to the basedioptric power and along which the lens functions for near vision,—anintermediate second tract (b) substantially parallel to the abscissa atthe value of the base dioptric power and along which the lens functionsfor far vision,—a peripheral third tract (c) consisting of aprogressively rising first part (c′) and of a second part (c″) parallelto the abscissa, said two parts (c′, c″) corresponding to the powers atwhich the lens functions progressively for intermediate distances in thefirst part and in a constant manner for near vision in the second part.2. A progressive multifocal contact lens in accordance with claim 1,characterized in that: the first tract (a) has a peak at the centre ofthe lens, which may have values up to about 6 dioptres, from which itdescends to a value of 0 dioptres, the equivalent of the base dioptricpower, at a distance from the centre of the lens that may range from 0.5to 1.5 mm; the second tract (b) is parallel to the abscissa at the levelof 0 dioptres and remains substantially constant from the foot of thefirst tract (a) to a point of union with the lower end of the thirdtract (c); the third tract (c) comprises: i) the first part (c′) thatprogressively rises from the point of union to a point situated at adistance from the centre of the lens between 2.5 and 3.5 mm and at adioptric power comprised between 0.5. and 5.0; ii) the second part (c″)that remains substantially constant at the value of the dioptric powercomprised between 0.5 and 5.0 for the whole of the remaining extensionof the optical zone of the lens, from the aforesaid distances from thecentre of the lens between 2.5 and 3.5 mm right through to the perimeterof the optical zone.